These are of the types which comprise:
(1) a central electronic unit mounted in the vehicle and powered by its battery, which can be activated to: activate or deactivate; PA1 (2) a device which, when activated, blocks the operation of the vehicle's engine; PA1 (3) a small; self-contained, battery powered transmitting unit, activated by a push button, carried by the user when he leaves the vehicle.
By pressing the push-button, the user sends a coded signal, at a single high frequency, which is received by the central unit. If the code is correct, the central unit will activate or deactivate the device which blocks operation of the vehicle engine.
Other systems are known, where an electronic key substitutes for the transmitting circuit in order to deactivate operation of the vehicle engine. Both of these systems are inconvenient to use.
With a transmitter activated system it is necessary for the user to consciously generate the signal which activates or deactivates the device which blocks operation of the vehicle engine. Therefore, if he forgets to consciously activate the antitheft system, the vehicle will be either partially or totally unprotected.
It is also possible for potential thieves to discover the code by monitoring the radio transmission from some distance, usually on the order of tens of meters, and recording the user's transmission when he uses his own transmitter.
The system activated and deactivated by the electronic key requires the conscious insertion and removal of the key itself in order to deactivate or activate the antitheft system, and if one forgets to remove the electronic key the vehicle will be unprotected.
UK-A-2051442 discloses a security system for a car comprising a control unit for installation in the car and a portable unit to be carried by the driver, which transmits, on recognizing a uniquely coded first signal transmitted by the control unit, a uniquely coded second signal to the control unit. The latter, on recognizing it cancels the disablement of car engine operation.
The security system above presents the drawback that it is also possible to discover the electronic key's code, using an easily obtained suitable measuring device, if one has access to the key itself for a few minutes.